1. Field of the Invention
This invention relates to a method for measuring magnetic or electric fields within a sample material and, more particularly, to a method using a scanning force microscope to track the topography of the surface so that such fields occurring close to the material surface are accurately measured at a small, constant distance from the surface.
2. Background Information
The measurement of a magnetic or electric field occurring within a sample material, from a measurement point traversing the surface of the material at a very close spacing beyond this surface, has been a subject of an ongoing effort by scanning probe microscopy researchers for several years. A measurement process of this type requires that a probe sensitive to magnetic or electric fields must be moved in a direction perpendicular to the sample surface during the traversing, or scanning process, so that the probe tracks the topography of the sample surface without contacting it. Moving the probe in this way is both important and difficult when the sample surface is quite rough, as is the surface of a number of materials for which this type of measurement can provide significant information, such as the surfaces of magnetic data storage media.
The scanning force microscope provides an accurate method for moving a probe along a surface in very close proximity thereto. A probe having a very sharp tip is moved along the sample surface being examined by means of a lateral actuator. The probe is mounted to a distal end of a cantilever, the proximal end of which is attached to a vertical actuator, which moves the probe tip into and out of engagement with the sample surface. Vibration in this vertical direction is applied to the distal end of the cantilever through the vertical actuator at a frequency close to the resonant frequency of the cantilever. The vibration of the probe tip at this frequency is measured. As topographical features of the sample surface increase the engagement of this surface with the probe tip, the probe tip vibration is decreased. As this engagement is decreased, the probe tip vibration increases up to a point at which the probe is freely vibrating out of contact with the sample surface. A feedback signal is generated as a difference between a signal representing probe tip vibration and a setpoint signal representing a level of vibrations occurring with the operational level of engagement desired between the probe tip and the sample surface. This feedback signal is used within a servomechanism loop including the vertical actuator to maintain the engagement at this operational level during lateral scanning.
However, when a single probe is used to track the surface topography, with a method such as that of the scanning force microscope, and simultaneously to track magnetic or electric fields, the signals produced by changes in topography tend to become mixed with the signals caused by these fields, so that accurate information cannot be recovered. What is needed is a way for separating the measurement of topography from the measurement of a field, while moving the probe in response to topographical variations during field measurements.